AIM ¶ 7-1-17 — Snow Drizzle Intensity
AIM 7-1-17 explains how snow and drizzle intensity (light, moderate, heavy) is estimated using prevailing visibility. Key thresholds for pilots.
In Plain English
AIM 7-1-17 gives pilots and weather observers a quick way to estimate the intensity of snow or drizzle based on prevailing visibility, since unlike rain, the rate of fall isn't easily measured by drop size or impact.
The thresholds are:
- Light: visibility more than 1/2 statute mile
- Moderate: visibility more than 1/4 SM up to 1/2 SM
- Heavy: visibility 1/4 SM or less
Why it matters operationally:
- Visibility-based intensity directly impacts whether you can legally and safely fly under VFR or shoot an instrument approach to minimums.
- Heavier snow or drizzle means rapidly changing conditions, potential icing, reduced braking action, and possible runway contamination.
- Understanding these criteria helps you correctly interpret METARs and SPECIs (e.g., -SN, SN, +SN) when making go/no-go and diversion decisions.
Note that the AIM is informational guidance, not a regulation — but these definitions align with how official surface weather observations report precipitation intensity.
AIM Source Text
FAA AIM ¶ 7-1-177-1-17. 7-1-17. Estimating Intensity of Snow or Drizzle (Based on Visibility)
Light. Visibility more than / 2 statute mile. Moderate. Visibility from more than / 4 statute mile to / 2 statute mile. Heavy. Visibility / 4 statute mile or less.
Oral Exam Questions a DPE Might Ask
Q1How is the intensity of snow or drizzle estimated?
Per AIM 7-1-17, intensity is estimated based on prevailing visibility rather than rate of fall.
Q2What visibility defines heavy snow or drizzle?
Per AIM 7-1-17, heavy snow or drizzle is reported when visibility is 1/4 statute mile or less.
Q3If visibility is 3/8 SM in snow, what intensity is reported?
Per AIM 7-1-17, that's moderate snow, since moderate corresponds to visibility from more than 1/4 SM to 1/2 SM.
Related Paragraphs in AIM Chapter 7